The invention relates to a process for the removal of HCN from gas mixtures that contain at least HCN and sulfur compounds, especially from gas mixtures that are obtained by partial oxidation or by gasification of carbon or oil, by catalytic decomposition of HCN, as well as a catalyst for the decomposition of HCN from a gas mixture by hydrogenation and/or hydrolysis.
Numerous hydrocarbon-containing gas mixtures, such as, for example, the gas mixtures that are obtained in the gasification of carbon or oil or other hydrocarbon-containing substances, must be freed of sour gas portions such as H.sub.2 S before they are further processed. H.sub.2 S removal is usually done, especially in the case of low H.sub.2 S contents, by gas scrubbing with a solvent that has a chemical or physical action. Since HCN that is contained in the gas mixture is dissolved by the solvents that are usually used and since HCN is not completely removed from the solvent when the solvent is regenerated, HCN is concentrated in the solvent, and in many cases the solvent is even decomposed by HCN. Therefore, HCN is removed by catalytic decomposition in a way known in the art before sour gas scrubbing.
A process for the removal of HCN in a catalyst that contains the elements nickel, uranium, and thorium in the form of their oxides from a gamma-aluminum-oxide carrier, is known from, for example, DE-OS 22 45 859. DE-AS 27 33 105 describes the catalytic hydrogenation of HCN in a catalyst that contains 10 to 45% by weight of hydrogenation-active metals of the sixth and/or eighth subgroup of the periodic system in the form of oxides and sulfides and 90 to 55% by weight of a carrier. The catalytic hydrogenation zone is operated there at a temperature of 100.degree. to 250.degree. C., and preferably 150.degree. to 230.degree. C.
It is known from EP-B-0 049 394 that the above-mentioned catalysts, on the one hand, bring about the hydrogenation of HCN but that in this case, on the other hand, undesirable secondary reactions occur. Thus, a considerable amount of methyl mercaptan is formed by hydrogenation of COS. At the same time, additional COS is subsequently supplied by the thio-conversion reaction. Moreover, the catalysts support the formation of formic acid from HCN. Methyl mercaptan (CH.sub.3 SH) and formic acid generally result in disruptions of the sour gas scrubbing slage.
Moreover, catalysts based on Al.sub.2 O.sub.3 are known as carriers that ensure the desired catalytic reaction of HCN without the above-mentioned undesirable secondary reactions, but only starting at temperatures of about 250.degree. to 300.degree. C. In the case of large amounts of gas, such as occur, for example, in the case of carbon gasification, especially combined cycle turbine power stations (gas and steam-turbine power stations), heating the gas mixture to temperatures of about 250.degree. to 280.degree. C. for HCN decomposition results in enormous costs. At the same time, high heat losses occur if the installation is not supplemented by appropriate, very expensive measures. In addition, acid components such as HF and HCl that are contained in the waste gas decompose in catalysts that are based on Al.sub.2 O.sub.3.